Introduction to Data Acquisition
Publish Date: May 29, 2014
Overview
This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series teaches you a
specific topic of common measurement applications by explaining the theory and giving practical examples. This tutorial gives an
introduction to the basic elements of a computer-based data acquisition system.
For the complete list of tutorials, return to the NI Measurement Fundamentals main page.
Table of Contents
1.
2.
3.
4.
5.
6.
7.
Introduction
Transducers
Signals
Signal Conditioning
Data Acquisition Hardware
Driver and Application Software
Relevant NI Products
1. Introduction
Data acquisition involves gathering signals from measurement sources and digitizing the signals for storage, analysis, and
presentation on a PC. Data acquisition systems come in many different PC technology forms to offer flexibility when choosing your
system. You can choose from PCI, PXI, PCI Express, PXI Express, PCMCIA, USB, wireless, and Ethernet data acquisition for
test, measurement, and automation applications. Consider the following five components when building a basic data acquisition
system (Figure 1):
• Transducers and sensors
• Signals
• Signal conditioning
• DAQ hardware
• Driver and application software
Figure 1. Data Acquisition System
2. Transducers
Data acquisition begins with the physical phenomenon to be measured. This physical phenomenon could be the temperature of a
room, the intensity of a light source, the pressure inside a chamber, the force applied to an object, or many other things. An
effective data acquisition system can measure all of these different phenomena.
A transducer is a device that converts a physical phenomenon into a measurable electrical signal, such as voltage or current. The
ability of a data acquisition system to measure different phenomena depends on the transducers to convert the physical
phenomena into signals measurable by the data acquisition hardware. Transducers are synonymous with sensors in data
acquisition systems. There are specific transducers for many different applications, such as measuring temperature, pressure, or
fluid flow. Table 1 shows a short list of some common phenomena and the transducers used to measure them.
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Phenomenon
Transducer
Temperature
Thermocouple, RTD, Thermistor
Light
Photo Sensor
Sound
Microphone
Force and Pressure
Strain Gage
Piezoelectric Transducer
Position and Displacement
Potentiometer, LVDT, Optical Encoder
Acceleration
Accelerometer
pH
pH Electrode
Table 1. Phenomena and Existing Transducers
Different transducers have different requirements for converting phenomena into a measurable signal. Some transducers may
require excitation in the form of voltage or current. Other transducers may require additional components and even resistive
networks to produce a signal. Refer to ni.com/sensors for more information on transducers.
3. Signals
The appropriate transducers convert physical phenomena into measurable signals. However, different signals need to be
measured in different ways. For this reason, it is important to understand the different types of signals and their corresponding
attributes. Signals can be categorized into two groups:
· Analog
· Digital
Analog Signals
An analog signal can exist at any value with respect to time. A few examples of analog signals include voltage, temperature,
pressure, sound, and load. The three primary characteristics of an analog signal are level, shape, and frequency (Figure 2).
Figure 2. Primary Characteristics of an Analog Signal
Level
Because analog signals can take on any value, the level gives vital information about the measured analog signal. The intensity of
a light source, the temperature in a room, and the pressure inside a chamber are all examples that demonstrate the importance of
the level of a signal. When you measure the level of a signal, the signal generally does not change quickly with respect to time.
The accuracy of the measurement, however, is very important. You should choose a data acquisition system that yields maximum
accuracy to help with analog level measurements.
Shape
Some signals are named after their specific shapes - sine, square, sawtooth, and triangle. The shape of an analog signal can be
as important as the level because by measuring the shape of an analog signal, you can further analyze the signal, including peak
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as important as the level because by measuring the shape of an analog signal, you can further analyze the signal, including peak
values, DC values, and slope. Signals where shape is of interest generally change rapidly with respect to time, but system
accuracy is still important. The analysis of heartbeats, video signals, sounds, vibrations, and circuit responses are some
applications involving shape measurements.
Frequency
All analog signals can be categorized by their frequencies. Unlike the level or shape of the signal, you cannot directly measure
frequency. You must analyze the signal using software to determine the frequency information. This analysis is usually done using
an algorithm known as the Fourier transform.
When frequency is the most important piece of information, you need to consider including both accuracy and acquisition speed.
Although the acquisition speed for acquiring the frequency of a signal is less than the speed required for obtaining the shape of a
signal, you still must acquire the signal fast enough that you do not lose the pertinent information while acquiring the analog signal.
The condition that stipulates this speed is known as the Nyquist Sampling Theorem. Speech analysis, telecommunication, and
earthquake analysis are some examples of common applications where the frequency of the signal must be known.
Digital Signals
A digital signal cannot take on any value with respect to time. Instead, a digital signal has two possible levels: high and low. Digital
signals generally conform to certain specifications that define the characteristics of the signal. They are commonly referred to as
transistor-to-transistor logic (TTL). TTL specifications indicate a digital signal to be low when the level falls within 0 to 0.8 V, and
the signal is high between 2 and 5 V. The useful information that you can measure from a digital signal includes the state and the
rate (Figure 3).
Figure 3. Primary Characteristics of a Digital Signal
State
Digital signals cannot take on any value with respect to time. The state of a digital signal is essentially the level of the signal - on or
off, high or low. Monitoring the state of a switch - open or closed - is a common application showing the importance of knowing the
state of a digital signal.
Rate
The rate of a digital signal defines how the digital signal changes state with respect to time. An example of measuring the rate of a
digital signal includes determining how fast a motor shaft spins. Unlike frequency, the rate of a digital signal measures how often a
portion of a signal occurs. A software algorithm is not required to determine the rate of a signal.
4. Signal Conditioning
Sometimes transducers generate signals too difficult or too dangerous to measure directly with a data acquisition device. For
instance, when dealing with high voltages, noisy environments, extreme high and low signals, or simultaneous signal
measurement, signal conditioning is essential for an effective data acquisition system. It maximizes the accuracy of a system,
allows sensors to operate properly, and guarantees safety.
It is important to select the right hardware for signal conditioning. You can choose from both modular and integrated hardware
options (Figure 4) and use signal conditioning accessories in a variety of applications including the following:
· Amplification
· Attenuation
· Isolation
· Bridge completion
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· Bridge completion
· Simultaneous sampling
· Sensor excitation
· Multiplexing
For more detailed information on these types of signal conditioning, visit Signal Conditioning Fundamentals for Computer-Based
Data Acquisition Systems.
Other important criteria to consider with signal conditioning include packaging (modular versus integrated), performance, I/O
count, advanced features, and cost. Use online tools at ni.com/signalconditioning to configure the best signal conditioning solution
for your application.
Figure 4. Signal Conditioning Hardware Options
5. Data Acquisition Hardware
Data acquisition hardware acts as the interface between the computer and the outside world. It primarily functions as a device that
digitizes incoming analog signals so that the computer can interpret them. Other data acquisition functionality includes the
following:
· Analog input/output
· Digital input/output
· Counter/timers
· Multifunction - a combination of analog, digital, and counter operations on a single device
National Instruments offers several hardware platforms for data acquisition. The most readily available platform is the desktop
computer. NI provides PCI DAQ boards that plug into any desktop computer. In addition, NI makes DAQ modules for
PXI/CompactPCI, a more rugged modular computer platform specifically for measurement and automation applications. For
distributed measurements, the NI Compact FieldPoint platform delivers modular I/O, embedded operation, and Ethernet
communication. For portable or handheld measurements, National Instruments DAQ devices for USB and PCMCIA work with
laptops or Windows Mobile PDAs (Figure 5). In addition, National Instruments has launched DAQ devices for PCI Express, the
next-generation PC I/O bus, and for PXI Express, the high-performance PXI bus.
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Figure 5. National Instruments DAQ Hardware Options
The newest DAQ devices from National Instruments offer connectivity over wireless and cabled Ethernet. NI Wi-Fi DAQ devices
combine IEEE 802.11g wireless or Ethernet communication, direct sensor connectivity, and the flexibility of NI LabVIEW software
for remote monitoring of electrical, physical, mechanical, and acoustical signals.
View the 6-minute NI Wi-Fi DAQ Guided Tour»
Figure 6. Wi-Fi Data Acquisition
6. Driver and Application Software
Driver Software
Software transforms the PC and the data acquisition hardware into a complete data acquisition, analysis, and presentation tool.
Without software to control or drive the hardware, the data acquisition device does not work properly. Driver software is the layer
of software for easily communicating with the hardware. It forms the middle layer between the application software and the
hardware. Driver software also prevents a programmer from having to do register-level programming or complicated commands to
access the hardware functions. NI offers two different software options:
· NI-DAQmx driver and additional measurement services software
· NI-DAQmx Base driver software
With the introduction of NI-DAQmx, National Instruments revolutionized data acquisition application development by greatly
increasing the speed at which you can move from building a program to deploying a high-performance measurement application.
The DAQ Assistant, included with NI-DAQmx, is a graphical, interactive guide for configuring, testing, and acquiring measurement
data. With a single click, you can even generate code based on your configuration, making it easier and faster to develop complex
operations. Because the DAQ Assistant is completely menu-driven, you make fewer programming errors and drastically decrease
the time from setting up your data acquisition system to taking your first measurement.
NI-DAQmx Base offers a subset of NI-DAQmx functionality on Windows and Linux, Mac OS X, Windows Mobile, and Windows
CE.
Application Software
The application layer can be either a development environment in which you build a custom application that meets specific criteria,
or it can be a configuration-based program with preset functionality. Application software adds analysis and presentation
capabilities to driver software. To choose the right application software, evaluate the complexity of the application, the availability
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capabilities to driver software. To choose the right application software, evaluate the complexity of the application, the availability
of configuration-based software that fits the application, and the amount of time available to develop the application. If the
application is complex or there is no existing program, use a development environment.
NI offers three development environment software products for creating complete instrumentation, acquisition, and control
applications:
· LabVIEW with graphical programming methodology
· LabWindows™/CVI for traditional C programmers
· Measurement Studio for Visual Basic, C++, and .NET
With LabVIEW SignalExpress, NI has introduced a configuration-based software environment where programming is no longer a
requirement. Using LabVIEW SignalExpress, you can make interactive measurements with NI Express technology.
7. Relevant NI Products
Customers interested in this topic were also interested in the following NI products:
LabVIEW
Data Acquisition
Signal Conditioning
For more tutorials, return to the NI Measurement Fundamentals main page.
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